Question

I am writing a call centre program in C# where multiple agents load the customers one by one from a table. In order to prevent more than one agent to load the same customer, I have added a new field to the table to show that the row is locked. When I select a row, I update that row and set the lock field to the ID of the agent who has selected that row. But the problem is during the time that I select the row and I lock it, another agent can select the same row since it’s not locked yet ! Is there a way I can handle this situation ? The database is MySQL 5 / InnoDB

Answer 1

Assuming you can only lock 1 profile per agent:

--Check for no lock
UPDATE T SET LockField = 'abc' WHERE ProfileId = 1 AND LockField IS NULL;

--Check to see if we updated anything. 
--If not, we can't show this row because someone else has it locked
SELECT ROW_COUNT();

Before i execute the update i have to select the id…

If you do the UPDATE in 1 statement, you don’t. We’re getting a little past my knowledge of MySQL syntax – but something like:

--Check for no lock
UPDATE T SET LockField = 'abc' WHERE ProfileId = (
   SELECT ProfileId FROM T WHERE LockField IS NULL LIMIT 1
);

--Check to see what we updated
SELECT * FROM T WHERE LockField = 'abc';

works pretty easily.

If you want to get a little more complicated (or MySQL doesn’t support the subquery), you can use an update lock with SELECT…FOR UPDATE:

START TRANSACTION;

--Put an update lock on the row till the xaction ends
--any other SELECT wanting an update lock will block until we're out of the xaction
SELECT @id = ID FROM T WHERE LockField IS NULL LIMIT 1 FOR UPDATE;

UPDATE T SET LockField = 'abc' WHERE ID = @id;

COMMIT TRANSACTION;